The Federal Communications Commission and cable television testing organizations such as Cable Labs, have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG algorithm or variations thereof. Such systems utilize other digital compression schemes, for example MUSICAM, to digitally code audio. MPEG is discussed in U.S. Ser. No. 774,006, filed Oct. 8, 1991 which is incorporated by reference herein.
Several systems, such as those demonstrated by the Advanced Television Research Consortium and the American Television Alliance, propose using quadrature amplitude modulation (QAM) in the transmission of the coded television information. QAM has demonstrated robust performance during the tests conducted by the ATTC (Advanced Television Testing Committee).
Conventional analog broadcasts (i.e. NTSC) provide the audio portion of the television signal as a frequency modulated signal while using amplitude modulation for the video signal. One desirable feature of NTSC is that a relatively clear audio signal can be received even when the video signal is marginally viewable.
Certain digital television systems, for example the DIGICIPHER system proposed by the American Television Alliance, utilize a single quadrature amplitude modulated carrier to provide both video data and audio data. An undesirable characteristic of such single carrier systems however, is that there is no gradual roll off in signal reception as with NTSC. Instead reception tends to abruptly "cut off" once the receiver-to-transmitter distance reaches the point where error correction and concealment processes cannot accurately compensate for the increased BER (bit error rate) and reduced C/N (carrier to noise).
One goal of another of the proposed digital television systems is to provide a transmitted television signal which imitates the gradual reduction in overall reception quality provided by conventional television signals. Toward this end, the Advanced Television Research Consortium has proposed quadrature amplitude modulating two separate carriers with separate data streams having different priority levels. One data stream provides video and audio information necessary to receive a "basic" television signal, and is transmitted on a first QAM carrier having the more robust transmission characteristic. A second data stream comprising information which provides the additional data necessary to receive the full HDTV signal is transmitted on a second QAM carrier having a transmission characteristic which is less robust. The aim of this prioritized transmission scheme is to provide for signal performance quality which would degrade gradually, rather than drop off abruptly with distance. The distinction between the gradual degradation between audio and video is incidental however since either (or both) data streams can comprise audio information as well as video. The twin QAM carrier approach is also more complex than single carrier QAM systems which do not try to emulate the gradual performance characteristics of conventional television signal.
It is therefore one object of the instant invention, to provide a single carrier QAM television signal which emulates the reception characteristics exhibited by audio and video in conventional analog television systems.
In an article entitled "Multiresolution Transmission for Digital Terrestrial Television Broadcasting", which is available from Philips Electronics, B. V., Eindhoven, the Netherlands, Paul G. M. de Bot describes multiresolution QAM techniques to enable the transmission of different levels of video using multiresolution signal constellations. De Bot discusses why this technique is preferred over the technique of time-division multiplexing different QAM constellations. In his article however, De Bot notes that an advantage of time-division multiplexing different QAM constellations is that any ratio of data rates can be used and the ratio can easily be changed during transmission as long as the total data stream data rate remains constant. De Bot does not however, discuss the advantages, or problems associated with, the use of multiresolution signal constellations or time-division multiplexing of different constellations to prioritize the transmission of audio separate from, and with respect to, video portions of a television signal.
In digital data transmission systems information is coded into data bits and a modem transmitter encodes groups of bits into symbols for transmission at a prescribed signaling rate. The analog transmission channels usually introduce linear amplitude and phase distortion to the transmitted signal as well as multipath. This distortion can cause an overlap of received symbols known as intersymbol interference (ISI). Such distortion can be compensated for by using an adaptive digital equalizer in the modem receiver to eliminate ISI caused by channel impairments. These equalizers require rapid, accurate and dependable estimation of the characteristics of the transmission channel in order to provide for fast start-up equalization.
The CCITT V.33 standard for point-to-point telephone modems described in the CCITT Recommendation V.33 "14,400 Bits Per Second Modem Standardized For Use On Point-To-Point 4-Wire Leased Telephone-Type Circuits" (Melbourne, 1988), incorporated by reference herein, describes a pseudo random noise (PN) training sequence preceded by a two point alternation sequence for a duration of 256 symbol intervals. The detection of the alternation sequence serves to detect the training sequence adjacent to it. The length of the CCITT alternation and training sequence combination is relatively long, but this poses no problem in telephone systems (or other dedicated two-way systems which inherently contain a "feed-back" loop for maintaining synchronization), because the alternation and training sequences are transmitted in burst fashion when communication commences and do not have to be frequently repeated. In television transmission systems however, where no "feed-back" loop exists, and where the television viewer may frequently tune among a number of simultaneously transmitted QAM signals, the CCITT alternation sequence and PN training sequence combination is too long for practical use. Another object of the invention is therefore, to provide an alternation sequence and training sequence suitable for use in television transmission systems.